As is known, a variable-format offset printing unit comprises three rotating cylinders the axes of which are oriented horizontally. The first cylinder, usually referred to as plate cylinder, carries, wound around it, a printing plate designed to transfer by image areas the ink received by a number of inking rollers. The second cylinder, usually referred to as intermediate or transfer cylinder, is covered with elastomeric material, such as for example natural rubber, and is placed in contact with the plate cylinder. The third cylinder, usually referred to as impression cylinder, is placed in contact with the intermediate cylinder. The web of paper or of other material to be printed is fed between the intermediate cylinder and the impression cylinder. In use, the intermediate cylinder receives ink from the plate cylinder by image areas that make up the printed image and transfers the ink on the web that is being fed between the intermediate cylinder and the impression cylinder.
A pair of cylinders consisting in a plate cylinder and an intermediate cylinder having the same diameter is called print format. In order to change the print length it is necessary to change the diameter of the plate cylinder and of the intermediate cylinder and therefore also to change the distance between the axes of the three cylinders. The variable-format offset printing units are therefore provided, in a known manner, with mechanisms for adjusting the distance between the cylinders, which mechanisms allow, in case of change in the diameter of the cylinders, in particular of the plate cylinder and of the intermediate cylinder (which define the print format and affect the print length), to suitably change the distance between the cylinders so as to ensure contact between the intermediate cylinder and the plate cylinder on the one hand and between the intermediate cylinder and the impression cylinder on the other. Furthermore, in order to ensure that the printing process is carried out in the right way, a given contact pressure must be kept between the cylinders of the printing unit that are in contact with each other, namely both between the intermediate cylinder and the plate cylinder and between the intermediate cylinder and the impression cylinder.
Document EP-A-2388140, which discloses a variable-format offset printing unit having the features set forth in the preamble of the enclosed independent claim 1, suggests for this purpose to support the plate cylinder in a fixed position, to support each of the axially opposite ends of the intermediate cylinder (or of the impression cylinder) at the end of a respective tilting arm that is hinged at the opposite end so as to be able to tilt about a fixed tilting axis, and to support each of the axially opposite ends of the impression cylinder (or of the intermediate cylinder) along a respective linear guide. In this way, while the axis of the plate cylinder is fixed, the axis of the intermediate cylinder (or of the impression cylinder) is movable along a circular path having its centre positioned on the fixed tilting axis of the hinges of the tilting arms, while the axis of the impression cylinder (or of the intermediate cylinder) is movable along a straight path defined by the linear guides. The tilting movement of the tilting arms on which the supporting members of the intermediate cylinder (or of the impression cylinder) are mounted and the translational movement of the carriage(s) on which the supporting members of the impression cylinder (or of the intermediate cylinder) are mounted are driven by means of respective actuation devices. In case of change in the print format, and hence of replacement of the plate cylinder and of the intermediate cylinder, the tilting arm and the supporting member must be suitably moved by means of the respective actuation devices so as to bring the intermediate cylinder in contact with the plate cylinder and the impression cylinder in contact with the intermediate cylinder with the desired contact pressure. A disadvantage of this known solution is that any change in the position of the axis of the intermediate cylinder, due for example to the need to adjust the contact pressure against the plate cylinder, inevitably results in a change in the contact pressure between the intermediate cylinder and the impression cylinder and therefore forces the operator to intervene, by suitably controlling the actuation device associated to the impression cylinder, to restore the correct pressure between the intermediate cylinder and the impression cylinder.